Direct Characterization of Interaction Processes between Dislocations
and Grain Boundaries by in situ TEM Nanoindentation
Shun Kondo1, Tasuku Mitsuma1, Eita Tochigi1, Naoya Shibata1,2, and Yuichi Ikuhara1,3,4
1
Institute of Engineering Innovation, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
2
PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
3
Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya, Aichi 456-8587, Japan
4
WPI Advanced Institute for Materials Research, Tohoku University, Aoba, Sendai 980-8577, Japan
Interaction between dislocations and grain boundaries is thought to greatly
affects the dislocation behavior, and thus governs the mechanical properties of
polycrystalline materials[1][2]. Many efforts have been devoted to understand the
mechanism of dislocation-grain boundary interaction, and many microscopic models of
the interaction processes have been proposed. However, it is still difficult to directly
characterize the elementary processes of dislocation-grain boundary interaction through
experiments because the interaction proceeds dynamically and microscopically. In
recent years, in situ transmission electron microscopy (TEM) enables direct observation
of dynamic and microscopic deformation processes. By using in situ TEM
nanoindentation techniques, we can apply the compressive stress to an arbitrary area in
a TEM specimen, and directly observe deformation processes at nanometer scale.
Moreover, using the bicrystals as a TEM specimen, we will be able to observe the
interaction of dislocations with well-defined grain boundaries. In the present study, we
demonstrate the direct observation of dislocation-grain boundary interaction in
strontium titanate (SrTiO3) by using the in situ TEM nanoindenation techniques.
In this study, we started with characterization of dynamic dislocation behavior
within a grain during the nanoindentation. Here, we used SrTiO3 single crystal as a
model of in-grain region. Using the knowledge obtained from nanoindentation
experiments of single crystals, we performed direct observations of dislocation-grain
boundary interaction by using bicrystal specimens. As a model grain boundary, we
selected two kinds of grain boundary, {100} low-angle tilt and twist grain boundary (θ ~
1°). On the low-angle tilt grain boundary plane, the periodic edge dislocations are
introduced, while the screw dislocation network is introduced on the twist grain
boundary. Thus, we will be able to observe the interaction of lattice dislocations with
each type of grain boundary dislocation individually.
In situ TEM nanoindentation experiments were performed by JEM-2010
(JEOL Ltd.) operated at 200 kV, which is equipped with the double-tilt TEM
NanoIndenter holder (Nanofactory Instruments AB.). This holder consists of a fixed
indenter tip made of diamond and movable specimen which can be precisely controlled
by piezo actuator. During the nanoindentation experiments, the sequential TEM images
were recorded as a dark-field movie to highlight the dislocation by a video camera with
the frame rate of 30 fps.
FIG. 1(a) shows the captured image from the movie of nanoindentation to a
SrTiO3 single crystal. We inserted the indenter tip along the [001] direction and
observed the deformation processes from the [010] direction. As we inserted the
indenter tip, dislocations were introduced into the crystals from the specimen edge. Ex
situ dislocation analyses using g·b invisibility criterion revealed that the slip systems of
introduced dislocations belong to the <110>{110} family, which is consistent with the
slip system of SrTiO3 at room temperatures in the past reports[3]. From the in situ
observation and ex situ dislocation analyses, dislocation behavior during the
nanoindentation can be modeled as shown in FIG. 1(b). The dislocations are emitted
from the edge of specimen and propagate in a semicircle shape. Then, a part of
dislocation line intersects with the specimen surface, resulting in the separation of the
single dislocation segment into two segments. Each of the segments propagates to
±[100] direction as a near-screw dislocation[4].
Next, we carried out the nanoindentation experiments to the bicrystal
specimens including low-angle tilt/twist grain boundary. Taking account of the above
experiments, we prepared bicrystal specimens so that the grain boundary could stand
screw dislocations' way, and we attempted observing that the screw dislocations interact
with each grain boundary. In the case of low-angle tilt grain boundary, screw
dislocations crossed the grain boundary by intersecting with grain boundary edge
dislocations, resulting in kink formation on the screw dislocations and a jogs on the
grain boundary edge dislocation. However, in the case of low-angle twist grain
boundary, lattice screw dislocations piled up on the grain boundary plane and hardly
passed through the grain boundary plane. This can be explained as follow. In order to
cross the twist grain boundary, the lattice screw dislocations will have to drag the jogs
on the screw dislocation which is formed as a result of intersection with the grain
boundary screw dislocations. But, the jog dragging will need the formation of vacancies
or interstitial atoms, hindering the dislocation motion at room temperatures. Thus, screw
dislocations piled up on the low-angle twist grain boundary plane.
Acknowledgements
S.K. acknowledges the support by the Japan Society for the Promotion of Science
(JSPS) research fellowship for young scientists.
References
[1] E. O. Hall, Proc. Phys. Soc. B 64 (1951) 747.
[2] N. J. Petch, J. Iron Steel Inst. 174 (1953) 25.
[3] T. Matsunaga et al., Phil. Mag. Lett. 80 (2000) 597.
[4] S. Kondo et al., Appl. Phys. Lett. 100 (2012) 181906.
FIG. 1. (a)Dark-field TEM image during the TEM nanoindentation to SrTiO3 single crystal and
(b)Schematic illustrations of introduction mechanisms of lattice dislocations